In telecommunications systems, once the signal has been produced in the transmitter it is necessary to inject it into the transmission medium, in order that it reaches the receiver.
The way of carrying out the injection depends mainly on the transmission medium employed.
In the case of injecting signals onto the electricity grid, this can be carried out with inductive coupling units which are usually constituted by current transformers which are placed around the power conductors in which it is desired to induce the signal. In these cases the operation is based on the principle that currents which vary in time generate magnetic fields which vary in time and viceversa. In other words, the radiofrequency (RF) current it is desired to inject, produces a variable magnetic field (in time, and therefore a magnetic flux also variable in time) inside the ferromagnetic core of the coupling unit and, besides, the variation of magnetic flux around a conductor, induces therein a current proportional to this variation.
Based on this principle, it is possible to induce signals between conductors enclosed by the same magnetic core. Because there are high electric currents circulating through the power conductors, and therefore, the magnetic field strength around the cables is very high, the problem arises that if a ferromagnetic core is placed around the cables, the magnetic core can become saturated and unusable for carrying out the function of signal injection.
In the state of the art it is known that this effect can be overcome by including an air gap in the magnetic circuit so that the reluctance of the magnetic circuit is considerably increased and the magnetic flux is decreased dramatically. This implies an increase in the insertion loss of the signal, which is not desirable in communication applications, wherein the loss signifies an effective reduction in the range of the communications system.
To counteract this effect the usual practice is to lengthen the coupling unit in order to try to cover more flux, but this makes the coupling unit more bulky and more expensive, which is not desirable; and all this while seeking a compromise solution between size, air gap, and insertion loss.
A noteworthy precedent of the present invention, is constituted by the document U.S. Pat. No. 4,346,340, which discloses a method for improving the AC behaviour of an inductor such as a transformer or choke; the tendency of the inductor core to be saturated by a low frequency AC or DC current flowing in the winding of the inductor is reduced by detecting the magnitude of such currents and applying a compensating current to a control coil wound on the inductor core, the purpose being to produce a magnetic flux in the inductor core which opposes the component of flux which is tending to cause saturation. An inductor to which this method can be applied comprises a main coil, a magnetic core, a control coil means to detect the presence in this main coil of current at a frequency below a determined limit, and means to apply a control circuit to said control coil, which tends to compensate the flux produced by the detected current.
The present invention allows the inductive coupling to be carried out without saturation of the ferromagnetic core taking place and without additional insertion loss occurring.